Increased volumetric capacity of axial flow compressors used in turbomolecular vacuum pumps

ABSTRACT

A turbo-molecular vacuum pump of increased pumping capacity has parallel access toward two initial impeller-rotors. An additional annular space is provided around the periphery of the first rotor and the conventional first stator disc is omitted, thus creating an accelerated annual gas flow entering directly into the second rotor without an accumulation of pressure.

FIELD OF INVENTION

The invention relates to axial flow stages of rarefied gas compressorscommonly used in turbo-molecular vacuum pumps and, more particularly, tomodifications of rotor/stator arrangements at the inlet of such pumps.The purpose of the invention is to increase the volumetric flow rate, asrelated to the property called ‘pumping speed’ in high-vacuumtechnology. The basic idea may also be of use for very high altitudeaircraft for increasing the mass flow of rarefied air into the engine.

BACKGROUND OF THE INVENTION

Axial flow pumping stages used in turbo-molecular vacuum pumps areessentially rarefied gas compressors that pump and compress the gas by adisplacement process, resulting from sweeping the gas through angledblades attached to a rotating shaft. The peripheral velocity of theblades must be very high because the displacement process isnon-positive and back flow must be prevented as much as possible. Invacuum pumps, usually operating in molecular flow conditions, thisvelocity must be commeasurable to the normal thermal molecularvelocities of the pumped gases. This is necessary for effective captureprobability of molecules entering into the space between passing blades.

It becomes immediately apparent that the maximal pumping rate (pumpingspeed) will be limited by the maximum peripheral velocity of the pumpingblades. This in turn is limited by the strength of the material of theblades. Peripheral velocity is proportional to the product of thediameter of the rotor (the distance between outer edges of oppositeblades) and the rotational speed (RPM). To obtain highest possiblepumping speed, the first bladed rotor is placed near the inlet plane ofthe pump. To prevent backflow, the inner diameter of the pump body ismade to be very close to the tip of the blades. Thus, the pumping speedis limited for a given rotor diameter and given RPM.

A secondary limitation arises from lower peripheral velocity at the baseof the impeller blades because the diameter (or radius) is smaller atthat location.

Thus, for consideration of maximizing pumping speed, the blades cannotbe made too long, and the effectiveness of molecular capture will dependon the average peripheral velocity of blade surfaces. The pumping speedwill depend on this average and on the annular area of the inlet planetraversed by the pumping blades.

Accordingly, there is a need for improving the pumping speed withoutsubstantially changing the overall size, cost and power consumption ofthe pump.

SUMMARY OF THE INVENTION

According to the first aspect of the invention, a vacuum pump isprovided with multiple axial flow stages in series, consisting ofrotor/stator pairs placed at the inlet of the pump. Typically, theangles of the blades in the stator disks are opposite to the bladeangles of the rotors. Regardless of the shape and size of the pump inletstructure, the limitation of the pumping speed at the inlet plane of thefirst stage is the property that the invention desires to increase.Thus, additional annular space is provided at the periphery of the inletrotor through which gas molecules can have access to the second stagerotor. This additional flow, under molecular flow conditions, does notinterfere with the flow arriving from the exit side of the first rotor,because the molecules do not collide and are “unaware” of the presenceof others. In prior art operation mode, the presence of the first statorwill increase the pressure (or density), due to compression, which mayresult in backflow and negate the desired effect. Therefore, it isproposed to remove the first stator disk entirely and thus use the firstrotor only for the pumping speed effect and free it from producingcompression. This renders the first rotor to become an auxiliary capturemechanism which sends an annular molecular beam into the second rotor.The first rotor can be confined at its periphery by a thin annular guardwhich will prevent radial spreading of the exit flow. The pumping speedof the second rotor is increased by receiving an additional flow fromthe added peripheral access space. This flow is only limited by thewidth of the annular space and the distance of the second rotor to theinlet plane of the pump, i.e. by conductance of that passage.

Thus, the basic invention provides an arrangement of sharing the initialgas flow capture among the first two rotors. Depending on detaileddesign, as much as 50% improvement in pumping speed can be achieved.Conversely, the blades can be made shorter to increase their averageperipheral velocity, which is desirable for efficient pumping of gaseshaving a low molecular weight. It is not effective to extend the newarrangement to the third stage rotor because of an additional loss ofcompression due to the omission of stator discs and due to the resultantlonger distance to the inlet plane of the pump. Conductance of annularcylindrical conduits rapidly diminishes with the growing ratio of lengthto width of the duct.

The principle of the invention remains valid whether or not the firststator is entirely removed or kept in its usual place with any bladeangle, including tilted backward (i.e. at the same tilt as the rotorblades), or made to be 90 degrees to the plane of rotation. It alsoremains valid regardless what kind of impellers follow the first two (orthree) axial pumping stages, including axial-flow, or molecular dragimpellers (with Gaede, Holweck, Siegbahn type pumping channels), spiralor concentric, or the regenerative/centrifugal kind. It also remainsvalid whether or not the shield around the blades of the first rotor ispresent. It remains also valid regardless of the shape of the conduitleading from the inlet plane of the pump to the entry into the secondrotor; it can be a concentric or non-concentric annular cylinder, or itcan have a cross-section of elliptical, ovoid, or polygonal shape.

There are three preferred embodiments apparent. First, for a given sizeof the inlet flange (or inlet area of the pump), create as great aspossible peripheral access from the inlet plane of the pump to the inletarea of the second rotor. Second, use a smaller rotor, rotating at ahigher RPM to obtain a pumping speed equivalent to same size pumps madeaccording to the prior art. Third, enlarge the body of the pump near theinlet to provide a higher conductance passage to the vicinity of thesecond rotor. The use of a smaller rotor assembly reduces the weightwith advantages of lower rotational moment of inertia and lower load forthe bearings. This arrangement can also be achieved by placing the firstrotor (as an auxiliary rotor attached to the main shaft) above theconventional inlet plane of the pump, protruding into the vacuumchamber, as long as sufficient space is available for pumped gas toenter directly into the second rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the accompanying drawings, which are incorporated by reference and inwhich:

FIG. 1 is a simplified cross-sectional schematic diagram of the inletsection of a turbo-molecular vacuum pump according to the prior art.

FIG. 2 is simplified cross-sectional schematic diagram of aturbo-molecular vacuum pump in accordance with an embodiment of theinvention.

FIG. 3 is a cross-sectional drawing demonstrating typical arearelationships of annular gas access channels in accordance with theinvention.

FIG. 4 is a simplified cross-sectional diagram showing an enlarged inletpart of the pump body, in accord with the second embodiment of theinvention

FIG. 5 is a schematic diagram showing an additional feature of theinvention where longer blades are used in the first rotor to enhance gascapture capacity without increasing the projected diameter and theperipheral velocity.

DETAILED DESCRIPTION

A simplified cross-sectional diagram of a conventional turbo-molecularpump is shown in FIG. 1. The inlet plane (110) defines the plane wherethe rarefied pumped gas enters the pump and where the onset of pumpingmechanism occurs by the action of the first bladed rotor (111).Compression occurs when the accelerated gas enters the stationary bladeddisk, the stator (112). There follow several sets of rotor-stator pairs(connected in series) to continue the compression process. The pumphousing body (113), enclosing the spacers (114) between stages, isplaced in very close proximity to the rotors in order to avoid backflow.The compressed gas is exhausted at (118) where connection is typicallymade to a different type of pump that is more effective at higherpressures and can exhaust directly to atmosphere. The pumping stagesthat follow the axial pumping section can be of any type. The entirerotor assembly (115) is attached to the shaft (116) which turns at highrotational speed around the axis (117).

The basic embodiment of the invention is shown in FIG. 2, where apassage of adequate conductance (210) is created at the periphery of thefirst rotor (211), to provide pumped gas access into the second rotor(212). The first stator (112, in FIG. 1) is absent to eliminatecompression, which is undesirable at this point. An additional axialdistance between the first and second rotors can be made as necessaryfor adequate access for entry into the second rotor (212). Thestationary annular ring (213) placed in proximity of the blade tips ofthe first rotor (211) can be omitted but may be preferably used toprevent spraying of the pumped gas in centrifugal directions. In thisarrangement, according to the invention, the capacity of capturing thegas molecules entering the inlet plane of the pump (110, in FIG. 1) isshared by the two leading rotors, while in a conventional (prior art)pumps, this capacity is entirely dependent on the performance of thefirst rotor alone.

FIG. 3 illustrates the comparative cross-sectional areas, at the inletplane of the pump of the active and inactive elements. The active areaof the rotor is the bladed section (310), where collisions occur betweengas molecules and rotating blades. The inactive area which is uselessfor pumping effect is the circle (311), where the peripheral velocitiesare too low for effective pumping action. The additional annular accessconduit (312) for reaching the second rotor is shown at the outerperiphery of the figure. The width of this duct should not be greaterthan is adequate for the desired improvement of overall pumping speed.Making it too wide will quickly reach diminishing returns. This spacecan be created, for example, by omitting the few spacers used for properaxial positioning of stators and replacing them by a thin cylindricalspool (214 in FIG. 2). The pump body position is outlined by the circle(313).

FIG. 4 is a schematic representation of the modified entrancearrangement according to the second embodiment of the invention. Thespace between the first rotor (410) and the second rotor (411) isenlarged to provide an adequate access (413) for the gas enteringdirectly toward the second rotor. The conventional first stator disk isomitted. The blades of the first rotor are surrounded by a thincylindrical ring (412) which is supported from the body by a few straps.This thin cylinder can be in the form of a converging or diverging coneor it can have a curved shape. In addition, such an arrangement providesthe possibility of using a smaller pump with an enlarged inlet bodysection that may reach a pumping speed of the next larger conventionalpump.

FIG. 5 is schematic diagram of the inlet section of a pump withelongated blades in a conical arrangement. This permits to shift alonger section of the blades into the region of a greater peripheralvelocity without enlarging the projected rotor diameter. The resultingadditional bending forces, in such arrangement, must be compensated byreinforcing the blade structure. The outer edges of the blades (510) areimmediately below the inlet plane (511), the central section (512)rotating around shaft (513) is at a lower level. The conventional firststator is omitted.

1. A high-vacuum pump containing an axial flow compressor as used at theinlet of turbo-molecular vacuum pumps, comprising: a housing having aninlet port and an exhaust port; a rotor containing impellers, attachedto the same rotating shaft, having inclined blades; one or moreadditional axial flow stages located within the housing, followed bypumping stages operating at higher pressures, characterized by providingaccess for the pumped gas toward two inlet rotors operating in parallel,each having inclined blades which provide the pumping mechanism; and amotor to rotate said impellers such that the gas is pumped from saidinlet to said exhaust port.
 2. A high-vacuum pump as defined in claim 1,wherein the leading inlet rotor impeller, having preferably a 45 degreeblade angle, has space around its periphery to provide access for thepumped gas to the second rotor-impeller thereby increasing the molecularcapture probability at the inlet of the pump and where the conventionalfollowing stator disk is either entirely omitted or has blade angleswhich do not impede the accelerated gas molecules leaving the firstrotor from passing directly toward the second rotor.
 3. A high-vacuumpump as defined in claim 2, wherein a thin cylindrical shield, havingconverging or diverging or curved shape, is placed in close proximity tothe first rotor to prevent spreading pumped gas molecules in lateraldirections and help direct the gas exiting the first rotor toward thesecond rotor.
 4. A high-vacuum pump as defined in claim 1, wherein theinlet section of the pump is enlarged to accommodate the additional,preferably annular, space for an increased conductance access for pumpedgas toward the second impeller.
 5. A high-vacuum pump as defined inclaim 1, wherein the first inlet rotor is shaped such that the impellerblades are elongated being placed in an arrangement of an invertedtruncated cone so as to increase the area of interaction between thepumped gas and the pumping blades.
 6. A high-vacuum pump as defined inclaim 1, wherein the first rotor can be placed as an auxiliary rotor,attached to the main rotating shaft, above the conventional inlet plane,protruding into the vacuum chamber where the connecting tube between thepump and the chamber has a larger diameter than the pump inlet port. 7.An axial flow compressor as defined in claim 1, used in applications forpumping rarefied gases, such as air in high-altitude aircraft in orderto increase the mass flow of air into the engine.